The invention relates to an adaptive material of ternary composition. It can be used advantageously in electric cables and their terminations where it serves to improve charge flow characteristics and reactions to transient electric fields.
The solution known in the prior art, e.g. as used in patent application WO 97/26693, is to introduce particles having non-linear electrical behavior, e.g. particles of doped zinc oxide, into the insulating material so as to improve electric field distribution therein. Nevertheless, that solution has the defect of possessing electrical characteristics that are difficult to reproduce. It is thus not possible to obtain behavior that is constant within composites made of such a matrix and such particles. For example, the elastic behavior of certain matrices can give rise to cavities around the particles, so electric characteristics are then no longer homogenous, which can give rise to undesirable local phenomena. In order to solve those problems, the recommended solutions consist of modifying the proportions and the quantities of particles so as to optimize the electric characteristics of such materials. Nevertheless, the desired level of reproducibility has not been achieved.
The invention enables those problems to be remedied by adding a new component to the known mixture, which component is to be found in the vicinity of the particles and is capable of making the electric field locally uniform within the composite.
The invention has three components.
The first component is a matrix of the kind commonly used in applications such as cable joints and terminations.
The second component is a collection of particles having non-linear electrical behavior. These particles are dispersed in uniform manner within the matrix. On coming into contact with an electric field, this component behaves in intrinsically non-linear manner and can reduce the electrical stress within the insulating matrix. It also possesses the property of being insulating in the absence of electric voltage. Doped zinc oxide can be used for this component.
The last component serves to provide bonding between the first two components. It is selected from surface-active agents. Its function is to fill as well as possible the interface between the matrix and the particles and to make the local electric field uniform. By way of example, this component can be polyethylene oxide (PEO) or polypropylene oxide (PPO). These elements can advantageously be used in a form that has been doped with a monovalent Li+ ion. Electrically-conductive polymers such as polyaniline or polypyrrol can also be envisaged. These examples are not limiting and the person skilled in the art can easily use other compounds that possess the same properties.
In the manufacturing process, the three ingredients are blended together while in powder form. It is possible to add a doping agent such as organic salts of lithium. It is conventional to use active agents to disperse the powder over the matrix. In the context of the invention, it is advantageous to use specific substances that also have conductivity properties, thereby avoiding an electric gradient that is too great at the interface between the matrix and the particles. Such substances are both surface-active agents and conductors. Conductive polymers can possess certain groups which have affinities with the matrix and with oxides of zinc. For example, with sulfonated polyaniline, it is possible to transform it into a suitable interface by adjusting the molecular mass and the viscosity of these substances. It is necessary to avoid the wetting agent being too liquid. If it is too liquid, then good dispersion will not take place and performance will again be random, thus preventing reproducibility. It is thus necessary for it to have viscosity close to that of the matrix. If viscosity is measured on a logarithmic scale, the viscosity of the wetting agent should be about one decade smaller than that of the matrix.
The invention makes it possible to improve the performance of the materials, and in particular the reproducibility of their electrical characteristics, and finally, it makes it possible to increase the number of matrices used and to obtain only a small amount of dispersion in the threshold stress. The concentration and the size of filler varies as a function of the application, thus making it possible to adjust the threshold stress. Advantageously, it is thus possible to use this material in applications where energy is small. In addition, the reduced amount of dispersion makes it possible to reduce the number of particles that are required.